Such a vessel is described in particular in international patent application publication WO 2006/037663 to the Applicant and comprises two side hulls connected to one another at each of their ends and at an upper part by cross-beams, and a mobile platform located between the two side hulls.
The mobile platform is a sealed caisson having an upper part capable of withstanding loads. Mechanisms are provided to vary the position of the mobile platform in relation to the two side hulls, between an upper position, in which the vessel floats solely via the two side hulls, and a lower position, in which the vessel floats via the platform and the two side hulls.
With the platform in lower position, the vessel converts to a landing craft (landing craft mode) and the vessel's draft is reduced by the additional buoyancy of the platform. This landing craft mode allows approaches to beaches, in particular, or to river banks in very shallow waters and, for example, allows the disembarking of equipment and military or non-military personnel on non-developed shorelines.
When the platform is in upper position, the vessel converts to a catamaran (catamaran mode) allowing the vessel to travel at high speed on its profiled hulls even in rough seas.
The mechanisms varying the position of the mobile platform between the upper and lower positions comprise hoisting means, possibly including one or more pairs of hydraulic cylinders located on each side of and along the platform. The hydraulic cylinders are hinged between the side hulls and the mobile platform of the vessel. The hydraulic cylinders can be actuated by control means, such as hydraulic pumps, which may be driven by geared motor assemblies.
In addition, the vessel is provided with a maneuvering and propulsion device, arranged symmetrically in the two side hulls, substantially at the bottom of the hulls, comprising two motorized propulsion systems, including hydrojets with steering and reversing mechanisms, ensuring good maneuverability of the vessel.
Hydrojets ensure a good propulsion yield, about 0.55 to 0.65 for vessel travel speeds ranging between about 30 and about 45 knots, with a water intake located underneath the corresponding side hull of the vessel. Each hydrojet is capable of operating in very shallow drafts.
However, hydrojets provide a less good propulsion yield, of between about 0.35 and 0.5, at slower navigating speeds of the vessel, speeds ranging from about 12 to about 25 knots. Each hydrojet suffers from the cavitation phenomenon at low speeds unless its dimensions are significantly increased to avoid this phenomenon. The increase in dimensions is detrimental to the overall weight of the vessel.